Millions of the slimy monsters are crawling our way and threatening to overrun our gardens and even our homes, as doors and windows are left open during the sweltering heat.

Experts are warning that the soft-bodied invertebrates "breed like rabbits", churning up to a staggering 400 EGGS at a time.

The outsized gastropods - which can grow up to half a foot long - spread their slime and feast on dead matter and dog excrement. They are particularly hardy and resist efforts by pest control to eradicate them, sometimes devouring up to 20 slug pellets before they even start to feel poorly.

Dr Ian Bedford, head of entomology at the John Innes Centre in Norwich, first encountered the slugs in East Anglia in 2012. He counted 350 of them in just one day and when he saw some eating a dead mouse he decided to send samples for identification.

The new breed of Spanish slug [Alamy]

The Spanish slug is a voracious predator that can survive eating many of the slug pellets that are supposed to kill them

Dr Ian Bedford, head of entomology at the John Innes Centre in Norwich

There were scattered reports of more sightings in April last year, but a sudden cold snap either killed many off or slowed down their breeding.

Yet the current heatwave – in which the mercury is set to top 91F by the end of the week - looks set to spark a breeding frenzy.

Dr Bedford said: "With the mild winter climate and the conditions we have seen in the spring, we are expecting them to make a comeback this year - I am starting to find a few dozen in the garden already."

The current heatwave is sparking a breeding frenzy among slugs [Matt Scott-Joynt/M&Y News Agency]

The scientist and his team of researchers are seeking to secure funding for urgent research, so as to tackle the monster molluscs before they destroy many crops. It is also feared that they could carry germs and disease.

These slugs, which are cannibalistic and eat others of their kind, are thought to have first arrived in Britain on imported lettuce and other salad leaves. Experts fear they will breed with native species - which play a vital role in our ecosystem - to create a mutant version that could resist all forms of pest control.

When the species first appeared in Scandinavia several years ago, they bred so quickly that squashed slimy slugs on roads caused multiple car crashes.

Common garden slug [Getty]

Our own native slugs play a vital role in the ecosystem and of the 30 species in the UK only four are classed as pests. The majority of our slugs actually live underground and the ones which can be seen feeding on the surface make up less than five per cent of the native population.

Dr Bedford warned: "The Spanish slug is a voracious predator that can survive eating many of the slug pellets that are supposed to kill them."

His team want people finding any of the beasts to report them at www.slugwatch.co.uk